Scaling of the Mammalian Brain: the Maternal Energy Hypothesis

T he concept of scaling is now well established, and allometric analysis has become a standard tool for quantitative comparison of features between organisms of differing body size. This technique has been applied with great success in the field of comparative physiology, notably to respiratory physiology of mammals and of the avian egg, and the major findings have been succinctly reviewed by Knut SchmidtNielsen (14). Comparative physiologists have repeatedly used scaling analyses to explore functional relationships, but in other fields the main objective has been to identify general scaling trends to exclude body size effects from comparisons between species. Thus various studies of comparative morphology, reproductive biology, and behavioral ecology have focused on distinctions between taxonomic groups and associated evolutionary relationships, without much concern for underlying functional relationships. Many authors trace the origins of the modern concept of allometric scaling back to Julian Huxley’s pioneering treatise on quantitative aspects of growth (6), but the first significant application of the concept of allometric scaling was in fact made in studies of mammalian brain size at the turn of the century by Eugene Dubois (3), the discoverer of Homo erectus. Dubois introduced and applied the standard allometric formula that is now almost universally used in scaling studies. Scaling relationships broadly conform to the power function Y = kX” (cu being the scaling exponent and k the scaling coeffi-